Question

Suppose that $20$ pillars of the same height have been erected along the boundary of a circular stadium. If the top of each pillar has been connected by beams with the top of all its non-adjacent pillars, then the total number beams is :

• A. 210
• B. 190
• C. 170
• D. 180

Answer 1

Answer: (C)

170

Answer 2

Total cases = number of diagonals $= {^{20}C_2} - 20 = 170$